Floodwaters from Tropical Storm Bonnie

Bonnie faced a tough environment for organized storm formation as she crept towards the Carolina coasts, with strong southerly wind shear and dry coastal air constantly trying to rip the storm apart. By the time Bonnie made landfall she was, in fact, a tropical depression. This was no consultation for the people living and vacationing near the North and South Carolina coasts, however, in the midst of the busy Memorial Day weekend. Two deaths by drowning due to strong rip currents generated by Bonnie’s sustained >40 mph winds as well as over $600,000 in damages left a shadow over what should have been a relaxing long weekend. Below is the storm track for Bonnie, courtesy of the Plymouth State Weather Center, showing its landfall along the South Carolina Coast.

The bands of convective storms swirling around the center of Bonnie created a large swath of heavy precipitation across the region just north of Savannah, GA and to the west of Charleston, SC. The maximum amount of precipitation that fell over the entire period in which Bonnie produced rainfall (here defined as the 72 hours from 7am May 28 – 7am May 31) was 11.84 inches. This rainfall was mostly concentrated over two areas, shown below: south of Statesboro and south and west of Hampton. Also plotted below is the MetStorm-generated precipitation mass curve plot at the area of largest recorded total rainfall, which was just north of Ridgeland, SC. The striking thing about this plot is the quick ramp-up of precipitation as Bonnie approached land, culminating in more than 2 inches per hour in the early morning of May 29th. Precipitation intensity here and throughout the area quickly fell off to a continued light rainfall for the next couple days as Bonnie turned and ventured out to sea once again.

The average recurrence interval, or ARI, is widely used to convey the rareness of rainfall events. An ARI is the probability of the occurence of a recorded rainfall amount over a specified duration in a given year. Below, the 6-hour ARI value for rainfall is plotted over the Bonnie storm area. The largest value of 211.76 years, again near Ridgeland, therefore means that at this point the largest 6-hour rainfall total produced by Bonnie is expected to occur on average every 211.76 years. Many areas effected by Bonnie saw 6-hour ARIs of over 25 years: no small event.

Flooding near Ridgeland caused the city’s wastewater treatment plant to overflow, dumping close to 100,000 gallons of wastewater into the nearby river system, and as with most storms, pictures from the public describe in detail extreme events in a more familiar manner than data can provide. Bonnie, along with Hurricane Alex before her, have signaled an early start to the Atlantic hurricane season.

photo courtesy of twitter user @cookies4monster

Please note that the maps presented here are preliminary and will be updated when new data become available. If you are interested in this product, or any other product from our MetStorm Precipitation Analysis tool, please email us or send us a message though our contacts page here.

-MetStat Team

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